There is known a reference voltage source comprising two closed control loops, each including a compensation DC voltage stabilizer of opposite polarities, which are alternately connected to the output of the source (cf. U.S. Pat. No. 3,458,723).
The source of the aforementioned patent features a low output resistance. On the other hand, it requires the use of time-stable resistors in the divider circuits to ensure a high time stability of the source's output voltage, keeping in mind that the error of the resistor dividers is proportional to the error of the source as a whole. The design of this square-wave voltage is such that it is hard to control the constant component of the output voltage or use electric signals to control the output voltage amplitude within a broad range.
Another known AC voltage source includes an amplitude-controlled sinusoidal voltage generator, a reference square-wave AC voltage source, a thermoconverter to which are alternately applied the output voltage of the generator and the voltage of the reference voltage source, and a detector intended to detect the difference signal at the output of the thermoconverter, which controls the amplitude of the sinusoidal voltage generator (cf. U.S. Pat. No. 3,484,705 or F. L. Hanson, "High-Accuracy AC Voltage Calibration", Hewlett Packard Journal, vol. 19, No. 10, June 1968).
The aforementioned design accounts for a high accuracy and good stability of the output sinusoidal voltage, due to the stable voltage supplied by the reference voltage source. On the other hand, it is impossible to provide a square-wave voltage at the output of the source, although that may be necessary for certain applications. Another disadvantage of this source that it limits the short-time stability of the generator's output voltage because the noise at the input of the detector is considerably amplified due to the use of the deep negative feedback in the system.
Another known reference voltage source comprises a bridge circuit which contains avalanche diodes in a first pair of opposite arms and resistors in a second pair of opposite arms, as well as two operational amplifiers alternately connected to the source's output terminal.
The inverting input of the first operational amplifier is connected to the anode and its output is connected to the cathode of one of the avalanche diodes. The inverting input of the second operational amplifier is connected to the cathode and its output is connected to the anode of the second avalanche diode of the bridge circuit (cf. USSR Inventor's Certificate No. 445,037, Cl. G05 F, 1/56).
The latter reference voltage source is disadvantageous in that it does not permit simultaneous control of the constant component of the output voltage and the amplitude of the square-wave AC voltage.